The present invention relates to power amplifiers, such as, but not limited to, those used in an apparatus for the identification of different materials by the use of nuclear quadruple resonance (NQR). NQR technology uses radio frequency power amplifiers operating between 0.3 MHz and 5 MHz to produce a magnetic field for excitation of the material under investigation.
The nuclear quadruple resonance detection is a technique able to identify different materials uniquely. This technology is useful for (but not limited to) explosives detection. It is based on the physical property that some nuclei with a spin of ½ or more possess an electric quadruple moment. The interaction of this quadruple moment with the residual electric field gradient of the crystalline structure results in energy states separated by energies accessible to excitation by radio frequency magnetic fields in the range 0.3-5 MHz. The amplitude of the induced echo signals is very small and depends within some limits upon the excitation magnetic field. This is the reason why practical NQR detection systems use a coil with high quality factor (Q) connected to the output of the radio frequency power amplifier for generation of the magnetic field.
Typically class A-B radio frequency power amplifiers are used for driving NQR coils at power levels up to 0.5-2 kW. Increasing the output power above these levels is desirable to increase the signal to noise ratio (SNR) of the NQR system, but leads to large and heavy power systems that cannot be used in the compact portable systems desired for convenient explosives detection. Class D amplifiers allow a significant increase of the amplifier power output while decreasing the overall physical size and weight. The modern power MOSFET transistor, with very small Ron on the order of 10 mΩ, allows a significant reduction of the energy losses in the transistor when used in a switching mode. The class D power amplifier applied to NQR technology allows the increase of output peak power to the region of tens of kilowatts.
A variety of different schematics for class D amplifiers are presented in periodical publications and patents. Usually, the class D amplifier is devised using the output transistors in a push-pull or H-bridge scheme. Typically, these power amplifier units employ MOSFET transistors coupled in parallel to achieve output power in the multi-kilowatt range. The output stages of such a power amplifier can also be coupled in series through the use of summing output-transformers. Separate transformers for every switching stage all have a common secondary winding to provide a serial connection through all of the distinct switching stages to yield a voltage summing output.
An additional requirement of pulsed high-Q NQR systems is rapid damping of the energy from the antenna to provide the optimum conditions for registration of the echo signals in the data acquisition intervals between transmitted pulses. This operation is typically provided by separate Q-damper devices coupled to the load.